This is a genuine question. There's probably an explanation for this I haven't thought of, but it just occurred to me and I thought I'd post it here. I'm sure a central reason would be that Japan was going to surrender, while Germany wasn't at the time.
The only point of nuance is the fact that Japan was still actively committing genocide at the time and it did end that slightly faster than may have happened if they had waited for formal surrender but the US didn't care about that and the reason was simply to prevent them negotiating with the soviets.